If such 3rd party had no connection to the property and is absolutely an isolated individual who has sold the property to someone, then the transfer is illegal and is voidable at the option of the true owner of the Property.
However, if such 3rd party has any apparent authority to sell the property to another person, then the concept of Ostensible Transfer of Property u/s 41 of Transfer of Property Act would apply, protecting the interests of such transferee in good faith (having taken reasonable care on his/her part). The burden of proof would here lie on the transferee to show the measures of reasonable care taken or to show that the 3rd party transferor ensured that the true authority of transfering the property remained out of sight of such transferee.
However, if no such ostensible authority was present there, the sale would probably fall under the Fraudulent Transfer and shall be voidable at the option of the true owner.
please file the application for injunction against 3rd party if the sale deed is in proceeding. if he sale the property then file injunction against the subsequent purchaser to staying or stop further proceeding.
see the sec 16 of the rent control act with comentries.
A third party can not enter into a sale agreement, and the agreement cannot be registered, unless he can submit some tangible and valid evidence that he is legally entitled to enter into such agreement.